Tag Helpers: Create a 'cache' Tag Helper

[DamianEdwards]

A new Tag Helper that utilizes the conditional content mode (aspnet/Razor#221) to cache the output of its contents via ICache. Supports varied results via attributes. In the case of a cache hit, the contents would not even be executed.

Usage

<cache [vary-by=""]
       [vary-by-header=""]
       [vary-by-query=""]
       [vary-by-route=""]
       [vary-by-cookie=""]
       [vary-by-user=""]
       [expires-on=""]
       [expires-after=""]
       [expires-sliding=""]
       [priority=""]>

Arguments

Attribute Name	Type	Details
vary-by	string	A string to vary the result by. The value is used as part of the cache entry key
vary-by-header	string	A header name to vary the cached result by. The header value is used as part of the cache entry key.
vary-by-query	string	A comma-separated list of query string parameter names that the result should be varied by. The query string parameter values are used as part of the cache entry key.
vary-by-route	string	A comma-separated list of route data parameter names that the result should be varied by. The route data parameter values are used as part of the cache entry key.
vary-by-cookie	string	A comma-separated list of cookie names that the result should be varied by. The cookie values are used as part of the cache entry key.
vary-by-user	bool	A boolean indicating whether the result should be varied by user. The user name will be used as part of the cache entry key.
expires-on	DateTime	The exact date/time the cache entry should be evicted.
expires-after	TimeSpan	The time from right now that the cache entry should be evicted.
expires-sliding	TimeSpan	The amount of time from last access that the cache entry should be evicted.
priority	CachePreservationPriority	Specifies how items are prioritized for preservation during a memory pressure triggered cleanup.

Cache Entry Key Generation

The cache entry key will be generated by combining a number of elements, each separated by a hyphen -:

• the prefix "CacheTagHelper-"
• the TagHelperContext.UniqueId
• the values from the various vary-by-* attributes specified. Each one must be prefixed with a constant to avoid key/value clashes and include both the key and value from the underlying store (if applicable) with the format prefix([sourceKey1]=[sourceValue2],[sourceKey2]=[sourceValue3]). Attributes not specified should be omitted from the key.

Examples:
Url: /resource?page=2, HTML: <cache vary-by-param="page">Page @page results...</cache>, Key: CacheTagHelper-34980348710984387343556-param(page=2)

Url: /resource?page=2&category=45, HTML: <cache vary-by-param="page,category">Page @page results...</cache>, Key: CacheTagHelper-34980348710984387343556-param(page=2,category=45)

Header: My-Header=Hello, HTML: <cache vary-by-header="My-Header">Some content</cache>, Key: CacheTagHelper-34980348710984387343556-header(My-Header=Hello)

Url: /resource?page=2&category=45, Header: My-Header=Hello, HTML: <cache vary-by-param="page,category" vary-by-header="My-Header">Some content</cache>, Key: CacheTagHelper-34980348710984387343556-param(page=2,category=45)-header(My-Header=Hello)

Example

<cache vary-by-header="User-Agent">
    <div>
        Your browser is @Request.Headers["User-Agent"]
    </div>
    @(Component.Invoke<MyViewComponent>(1, "test"))
</cache>

[DamianEdwards]

A thought: this Tag Helper (along with #1553) doesn't actually have any dependency on MVC. It depends on more than Razor however (ICache for this one, IHostingEnvironment for #1553) so I'm not sure another logical location exists. Open to suggestions.

[yishaigalatzer]

We need to review the design for this prior to commencing coding with @loudej @davidfowl

[yishaigalatzer]

We are going to coordinate the design for this with #1544.

The main questions we have at the moment are

1. What gets cached (do we cache the string collection in the cache writer, or put it all in a large buffer).
2. How does it work with Flush (one of the current thoughts in flush was to break out individual buffers).

[pranavkm]

Follow up issue to support Flush inside the tag helper - #1819